CORS Response Header (Intermittently) not being returned in IIS - dotnet Web API on IIS 10

Question

I scrape websites for data and then post it to a c# .NET 6.0 to a IIS server running 10.0.17763.1. After some RANDOM amount of time by post is being rejected by CORS with the following message,

Access to fetch at '{myUrl' from origin 'theirUrl' has been blocked by CORS policy: Response to preflight request doesn't pass access control check: No 'Access-Control-Allow-Origin' header is present on the requested resource. If an opaque response serves your needs, set the request's mode to 'no-cors' to fetch the resource with CORS disabled.

The script I am using is

setTimeout(function() {fetch('myEndpointUrl', {
    method: 'POST',
    headers: {
        'Accept': 'application/json, text/plain, */*',
        'Access-Control-Allow-Origin': '*',
        'Content-Type': 'application/json'
    },
    body: JSON.stringify(
    {   
        portURL:   window.location.href, 
        portNumber: window.location.href, 
        portName:  document.getElementsByClassName("MuiGrid-root MuiGrid-item MuiGrid-grid-sm-true")[0].textContent
    }

The Program.cs is the following

using Kinetics.DatabaseContext;
using Kinetics.Models;
using Microsoft.EntityFrameworkCore;
using Microsoft.Extensions.Configuration;
using System.Configuration;
using Microsoft.AspNetCore.Cors;
using System.Web.Http;

var builder = WebApplication.CreateBuilder(args);

// Add services to the container.

builder.Services.AddControllers();
// Learn more about configuring Swagger/OpenAPI at https://aka.ms/aspnetcore/swashbuckle
builder.Services.AddEndpointsApiExplorer();
builder.Services.AddSwaggerGen();
//cors
var devCorsPolicy = "devCorsPolicy";

builder.Services.AddCors(options =>
{
    options.AddPolicy(devCorsPolicy, builder => {
        //builder.WithOrigins("http://localhost:800").AllowAnyOrigin().AllowAnyMethod().AllowAnyHeader();
        builder.AllowAnyOrigin().AllowAnyMethod().AllowAnyHeader();
        //builder.SetIsOriginAllowed(origin => new Uri(origin).Host == "localhost");
        //builder.SetIsOriginAllowed(origin => true);
    });
});

// registers dbcontext
builder.Services.AddDbContext<KineticsDataContext>(options =>
{
    options.UseSqlServer(builder.Configuration.GetConnectionString("sqlConnection"));
});

var app = builder.Build();
app.UseCors(devCorsPolicy);
// Configure the HTTP request pipeline.
if (app.Environment.IsDevelopment())
{

}
app.UseSwagger();
app.UseSwaggerUI();
app.UseHttpsRedirection();

app.UseAuthorization();

app.MapControllers();

app.Run();